index.css

*, *:before, *:after {
    box-sizing: border-box;
}

body {
    color: darkslategrey;
    display: flex;
    align-items: center;
    justify-content: center;
    background: antiquewhite;
}

h1 {
    font-size: 6rem;
    letter-spacing: 0.2rem;
    margin-bottom: auto;
}

p {
    text-align: center;
    font-size: 2rem;
}

button {
    background-color: -internal-light-dark(white, black);
    padding: 1rem 10rem;
    border-radius: 3rem;
    border: none;
    cursor: pointer;
}

.call-btn {
    background-color: darkslategrey;
    color: antiquewhite;
    font-size: 3rem;
    margin-left: 7rem;
}

.hangup-btn {
    background-color: darkred;
    color: white;
    font-size: 1.5rem;
    margin-left: 6rem;
    margin-top: 4rem;
}

.modal {
    padding: 5rem;
    background-color: whitesmoke;
    border-radius: 2rem;
    width: 40rem;
    height: 20rem;
}

.inner-modal {
    text-align: center;
}

.modal label {
    font-size: 1.5rem;
}
.modal input {
    margin: 1rem 7rem 3rem;
    display: block;
    padding: 1rem;
    border-radius: 3rem;
    box-shadow:0 0 15px 4px rgba(0,0,0,0.19);
    border: none;
    width:50%;
}

.connect-btn {
    background-color: #0c1d1d;
    color: whitesmoke;
    font-size: 1.5rem;
}